Wheellock

The Wheellock was the second major type of mechanical firearm action, after the matchlock. Wheellock firearms solved many of the issues of the matchlock, particularly the need to keep the match from burning out and the safety risks associated with handling black powder in the immediate proximity of a lit match, but were always too expensive and complicated to see widespread use. They were also the first type of firearm that could reasonably be concealed or holstered.

The mechanism functions like a cross between a toy spark gun and a cigarette lighter. Unlike most other types of lockwork, the pivoting arm which holds the piece of iron pyrite (called a dog but often incorrectly referred to as a cock in flintlock-era examples) is not connected to the trigger. Instead, the trigger operates a spring-loaded steel wheel inside the lock, which moves against the pyrite piece held by the dog to create sparks: the spring in the dog mechanism is only to hold it against the wheel when it is in the operating position, and what appears to be the cocked position (dog forward) is actually just the safe position. Iron pyrite was used in these weapons because it is softer than flint: while flint is better at generating sparks, using flint in a wheellock would compromise the lifespan of the wheel. Wheellock mechanisms were manually tensioned prior to use, using a projecting shaft that could be wound using a wrench or key in a similar manner to a clockwork mechanism.

(As a note, the arm on a wheellock is called a "dog" because it is a part that impedes ("dogs") the motion of another part. "Doglock" firearms (an early type of flintlock) are so called because they have an external "dog" safety catch that impedes the motion of the cock rather than internals that allow the weapon to maintain a half-cock position)

Note that nearly all firing Wheellocks in film or television are modern firing replicas due to original Wheellocks being unsafe to shoot and much more expensive.